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North American developments in herbicide tolerant crops
by Mike Owen

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November, 1998 -  This paper  was presented at the 1997 British Crop Protection Conference in Brighton, England.  It describes the implementation of herbicide tolerant crops in North America and presents information on the benefits and risks of this technology.


The use of herbicide tolerant crops in North America has focused primarily on corn, soybean, and canola. While there have been a number of herbicide tolerant crops available for several years, grower adoption of this technology has been somewhat slower than anticipated. However, with the availability of glyphosate-resistant soybeans and canola and imidazolinone-resistant corn and canola, and the marketing emphasis that companies will place on these products, utilization will likely increase dramatically. Objectively, when considering the use of herbicide-tolerant corn or soybeans, and the appropriate herbicide, weed management is not conceptually different than where traditional crop varieties and herbicide programs are used. However, farmer expectations are considerably greater with the new technology compared to existing crops and herbicide treatments. Farmers generally have failed to recognize the changes in management skills required to effectively use the herbicide tolerant crops. While the herbicide tolerant crops may have some advantages compared to current practices, their use also results in acceptance of risks associated with the technology.


Bridges (1994) suggested that weeds represent the most important pest complex and estimated that the impact of weeds on the United States economy exceeds $20 billion annually. Holt (1994) warned of a general lack of research to support the development of alternative weed management strategies and pointed out the problems of current herbicide use.

The use of herbicides has created considerable controversy in the United States. Strange and Miller (1994) suggest that the chemical dependency of modern agriculture has caused the decay of rural America and presented evidence that many farmers believe that there is too great a reliance on herbicides. Advocates of herbicide tolerant crops suggest that this technology is no different than that currently utilized for weed control, but does offer economic and environmental advantages (Burnside, 1996). Others strongly indicate that herbicide tolerant crops and the resultant use of herbicides are major environmental, economic and ecological concerns and recommend the implementation of sweeping restrictions to limit the adoption of the technology (Goldburg et al., 1990).

It is the intent of this paper to review the current status of herbicide tolerant crops in North American agriculture. The benefits and risks of the technology will be objectively reported and successes and problems associated with herbicide tolerant crops discussed. Several views will be taken; the implications of herbicide tolerant crops on weed management systems will be reviewed, how herbicide tolerant crops impact agriculture ecologically will be considered, and farmer expectations of herbicide tolerant crops documented. 


Industry has developed a number of herbicide tolerant crops including canola, sunflower, cotton, corn, and soybean. Research has been conduct on many more crop species and with a great number of herbicides (Dyer, 1996). Wilcut et al. (1996) suggest that herbicide-tolerant crops will be advantageous for weed management even if effective herbicides are already available. Do herbicide-tolerant crops represent a different strategy for weed management? Coble (1994) stresses the need for weed thresholds (economic thresholds) thus facilitating the use of herbicides only when needed to protect crop yields. Hess (1996) indicates that the risks associated with an integrated weed management system based on herbicide-tolerant crops are due to a lack of knowledge about the ecology and biology of the crop and associated weeds. Farmer expectations for weed control are greater than what the use of these strategies, in conjunction with herbicide-tolerant crops, will provide.

Experiences in the Midwest indicate that herbicide resistant weed populations are rapidly expanding and farmers are not managing the problem. Proponents of herbicide-tolerant crops suggest that this technology will improve management options to deter the development of herbicide resistance in weeds (Wilcut et al., 1996, Burnside, 1996). The author suggests that herbicide-tolerant crops and the resultant use of specific herbicides will increase the potential for the development of resistant weed populations, but the technology could also be used effectively to deter weed resistance. Even if, as suggested by the industry, there is little potential for resistance to glyphosate to develop in weed populations (Anon., 1997), selection will still occur resulting in weed populations that are not effectively managed by the herbicide (Radosevich & Holt, 1984).

Herbicide-tolerant crops do represent a potential weed problem to rotational crops. For example, corn hybrids that are tolerant to sethoxydim (SR varieties) have been difficult to manage in soybeans. However, with appropriate planning, volunteer herbicide-tolerant crops should be rather easy to management in rotational crops if the appropriate herbicide systems is used.

Ideally, herbicide-tolerant crops will improve the use of alternative weed management strategies. Wyse (1992) and Burnside (1996) suggest that herbicide-tolerant crop technology will increase the utilization of alternative strategies for weed management. However, a recent survey conducted by the Weed Issue Team at Iowa State University, and supported by the Leopold Center for Sustainable Agriculture, demonstrates clearly that it is unlikely that herbicide-tolerant crop technology will increase the use of alternative weed management strategies.


Farmers have expectations for weed control that are unreasonable from an environmental, ecological, and economic perspective. The herbicide industry has done an effective job of educating farmers about the level of weed control herbicides will provide and the consistency that this level of efficacy will be delivered. As a result, farmers have Alearned@ that weed control is synonymous with weed-free and a zero-tolerance for weed escapes now exists in much of the Midwest.

The use of herbicide-tolerant crops is thought by farmers to dramatically improve weed control and potentially reduce costs. Competition among seed companies and the demand for new products has resulted in claims that herbicide-tolerant crops will allow better and more effective weed control (Duvick, 1996). Glyphosate-tolerant crops are positioned as the answer to all weed problems (Anon., 1997). A new prepackage herbicide combination of imazethapyr and imazapyr is promoted to control woolly cupgrass (Eriochola villosa) when used in conjunction with imidazolinone tolerant corn hybrids. These claims are made because of a competitive market place and farmers expect that herbicide tolerant crops will control weeds more effectively. Attempts to control weeds at a level expected by farmers is possible, but not without increased economic and environmental risks.

The expectation by farmers that herbicide-tolerant crops will improve potential yields, and thus economics, by eliminating risks of weed interference is not likely to be realized. Economic models suggest that herbicide-tolerant crops will not likely impact the economics of crop production (Tauer & Love, 1989). Typically, the level of weed control provided by herbicide-tolerant crop management systems is equal, but no better than conventional systems. Farmers also expect weed control costs to be reduced when herbicide-tolerant crop management systems are used. Experience in Iowa suggests that the cost of weed control with the new technology is similar to existing systems.


Concerns have been expressed about the use of herbicide-tolerant crops and the increased risk of herbicide drift (Owen, 1994, Owen, 1997). However, given the high use of herbicides in agriculture, the increased potential for herbicide drift resulting from the use of herbicide-tolerant crops is minor. Drift is an inevitable consequence of current application techniques: all herbicides will drift. However, there is some concern associated with the herbicides that are used with herbicide-tolerant crops, and the increased number of applications likely necessary with some of these herbicides to meet weed control expectations. In Iowa, herbicide drift complaints were higher in 1997 than in past years (Charles Eckerman, Iowa Department of Agriculture and Land Stewardship, personal communication). The increased use of herbicide-tolerant crops and the resultant herbicide applications possibly contributed to this increase.


It has been argued the use of herbicide-tolerant crops and the resultant herbicides will increase the amount of herbicides applied to a field (Goldburg et al, 1990). However, given that the costs of the herbicides is similar to conventional systems, the concern for excessive amounts of these herbicides to be applied is unfounded.

However, herbicide carryover is a frequent problem in Midwest agriculture (Curran et al., 1991). Wrubel and Gressel (1994) estimate that 64% of the soybeans in the United State receive applications of ALS inhibitor herbicide classes. While the percentage corn treated with these herbicides is lower, the number is increasing rapidly. Given the residual characteristics of some ALS inhibitor herbicides and the lack of tolerance that rotational crops demonstrate to these herbicides, and the high usage, carryover is a significant concern. The use of herbicide-tolerant crops such as imidazolinone-tolerant corn hybrids and sulfonylurea-tolerant soybean provides an opportunity to manage the potential carryover problems and may be an excellent use of this technology (Owen, 1994).


While research has been conducted on a number of different crops, those that have engineered herbicide tolerance and currently with the greatest economic importance in North America are corn, soybean, and canola (Dyer, 1996, Re et al., 1996). One of the most important problems anticipated with the use of herbicide-tolerant crops was experienced in 1997. While it is intuitively obvious that only the herbicide-tolerant crop should be treated with the herbicide and not the sensitive crop, there were many examples of this type of mistake in North America. Corn that was not tolerant to sethoxydim was treated with sethoxydim, hybrids that were not imidazolinone-tolerant were treated with the prepackage mixture of imidazolinone herbicides, and glyphosate was applied to sensitive soybean varieties. In all cases, these mistakes were economically disastrous. There is a need for better communication between the applicator and the farmer.

Herbicide-tolerant corn

Currently in the Midwest, there are three herbicides for which engineered herbicide-tolerant hybrids exist; glufosinate-tolerant, imidazolinone-tolerant, sethoxydim-tolerant corn hybrids are commercially available. The imidazolinone-tolerant hybrids are most widely available and have the greatest marketing effort. Glyphosate-tolerant corn hybrids have been evaluated in the field for several years, but are not available for commercial use.

Glufosinate-tolerant corn, in conjunction with glufosinate, were positioned as a strategy to manage problem weeds and for use in no tillage production systems. Often, the glufosinate was applied in combination with a residual herbicide. Farmer success was varied, depending on the management skills and expectations. Performance of glufosinate-based systems in Iowa State University research was variable depending on the level of weed infestation and environmental conditions.

American Cyanamid has launched an aggressive marketing campaign for the use of imidazolinone herbicides in imidazolinone-tolerant corn and has registered a number of prepackage herbicide mixtures for use in imidazolinone-tolerant corn. American Cyanamid has actively positioned these products with a strong emphasis on the management of resistant weed populations which may result from the use of imidazolinone herbicides.

While these combinations include a herbicide with a different mode of action, with the exception of imazethapyr plus imazapyr, there is some question whether or not this strategy has value in reducing the potential for resistant weed populations (Wrubel & Gressel, 1994). The occurrence of imidazolinone-resistant weeds in Iowa increases and the use of imidazolinone herbicides in both corn and soybeans will contribute to the problem (Robert G. Hartzler, Iowa State Univ., personal communication).

The acceptance of imidazolinone herbicides for weed control in imidazolinone-tolerant corn has not been widely successful to date. However, the new combination of imazethapyr plus imazapyr demonstrates excellent activity on specific problems weeds, such as woolly cupgrass, that may increase farmer use of this technology. However, there has also been occasional fields of imidazolinone-tolerant corn that have exhibited herbicide injury at an unacceptable level. Observations at Iowa State University suggest that a number of factors are involved, including environmental stress and other agronomic characteristics of the specific hybrid. However, it is apparent that the cross-resistance to different imidazolinone herbicides may not be consistent and injury may occur from these herbicides applied topically at rates currently used.

The use of imidazolinone-tolerant hybrids has been suggested as a strategy that, in part, may lessen the impact of ALS inhibitor herbicide carryover. While the level of cross-resistance that is demonstrated by the tolerant hybrids may vary, it should be sufficiently high enough to lessen the occurrence of injury (Owen, 1997).

Sethoxydim-tolerant corn hybrids were available on a limited basis in 1996 and 1997. This technology was positioned as a strategy to control specific weeds such as woolly cupgrass, wild proso millet (Panicum miliaceum), and quackgrass (Agropyron repens). Generally, the results of this technology have been good, although often not as effective, given the biological characteristics of the target weed, to meet farmer expectations. Iowa State University positions the use of sethoxydim-tolerant corn hybrids and sethoxydim as an important component in a woolly cupgrass management program but not the answer.

Volunteer sethoxydim-tolerant corn from 1996 was a problem in 1997 soybean fields. These hybrids demonstrate some cross-tolerance to other aryloxyphenoxypropionate and cyclohexanedione herbicides and farmers how attempted to use these products did not control the volunteer weed at a level to meet expectations.

Herbicide-tolerant soybeans

Currently, there are three herbicides for which herbicide-tolerant soybean are commercially available. These include glyphosate and the sulfonylureas chlorimuron and thifensulfuron. Glufosinate-tolerant soybean varieties will be available in the near future. Of these, the glyphosate-tolerant soybeans has generated the greatest interest in farmers.

Glyphosate-tolerant soybean varieties are viewed by farmers as the answer to all weed management problems. Monsanto has positioned this technology in no tillage and narrow row spacing systems and farmers presume that it will eliminate all risks associated with weed control. In 1996, in Iowa planting was very late and, as a result most of the postemergence herbicide applications were applied in late June and July. At this time, most of the weed germination events had occurred and a single application of a postemergence herbicide was generally effective.

In 1997, planting occurred early and weed germination required an earlier application of the postemergence treatments. In many instances, unless alternative weed management was included, second and third applications were considered necessary by farmers. Glyphosate-tolerant soybeans allowed these later applications. Experiences in 1997 suggest that better management skills are required and often second applications may be needed for glyphosate-based weed control systems.

Most of the herbicides in the Midwest are applied by commercial applicators. Concerns about glyphosate drift may have seriously affected the timeliness of many applications in 1997. Further, the commercial applicators were expected to make the application timing decisions in many instances. The amount of time required to make these management decisions created a problem for the commercial applicators.

The issue of yield potential with glyphosate-tolerant soybean varieties was also a point of discussion. It is suggested that there is no loss of genetic yield potential with the glyphosate-tolerant soybean varieties (Harper, 1997) yet farmer complaints from 1996 experiences were in evidence. Further investigation suggests that many of the reported "low" yields were attributable to delayed glyphosate applications resulting in weed interference. As with all postemergence herbicide systems, management skills are important. The understanding of crop/weed interaction, the impact of the environment on plant development, and the implications of weed populations on potential crop loss are critically important for maximizing yield potential; these are not simple strategies!

One of criticisms about weed management programs based on a single herbicide is the potential for select resistant weed populations (Duke et al., 1991). Monsanto has suggested that due to the mechanism of action, weed resistance will not occur (Anon., 1997). Weed scientist have cautioned that regardless of whether or not resistance in weeds does develop, population shifts to weeds that are more tolerant to a particular herbicide or "avoid" the strategy are likely (Holt, 1994).

Experiences in Iowa during 1997 suggest that these population shifts can occur rapidly. Common waterhemp (Amaranthus rudis) populations demonstrate delayed germination and have "avoided" planned glyphosate applications. Velvetleaf (Abutilon theophrasti) demonstrates greater tolerance to glyphosate and farmers are reporting problems controlling this weed with the rates of glyphosate for which they are willing to pay.

Sulfonylurea-tolerant soybeans have been commercially available for several years. DuPont specifically created prepackage mixtures of chlorimuron ethyl and thifensulfuron methyl that had elevated rates thus improving the weed spectrum. On traditional soybean varieties, these prepackage mixtures did not provide sufficient crop safety. However, dramatic increase in common waterhemp across the Midwest and the occurrence of ALS inhibitor resistance in some of these populations may have limited the farmer acceptance of this system. Further, the implications of elevated chlorimuron ethyl rates, high pH soil types, and resultant concerns for herbicide carryover to rotational corn do not favor the use of sulfonylurea-tolerant soybeans and the prepackage herbicide mixtures. Regardless, these systems have been used to some extent in Iowa.

Another use of the sulfonylurea-tolerant herbicide may be to lessen the negative impact of prosulfuron carryover from corn to soybeans. Prosulfuron is a sulfonylurea herbicide marketed by Novartis for postemergence weed control in corn. Prosulfuron has a relatively long soil residual, particularly in high pH soil (Michael Johnson, Novartis, personal communication) and has caused injury and yield reduction to rotational soybeans. The use of sulfonylurea-tolerant soybeans instead of traditional varieties has promise to minimize this problem (Mark Vogt, Iowa State Univ., personal communication).

Herbicide-tolerant canola

Weeds represent one of the most important factors limiting canola production. A number of weeds are not effectively controlled by the available herbicides and changes in tillage and cultural practices are resulting in weed population shifts thus increasing the weed management problems (Darwent, 1994, Alan Green, American Cyanamid, personal communication). Herbicide-tolerant canola is thought to provide a solution to many of these problems.

Triazine-tolerant canola cultivars were developed thus allowing the use of triazine herbicides. However, these cultivars do not have the same yield potential as other cultivars and have not been widely used (Wall, 1992). However the development of glyphosate-tolerant, glufosinate-tolerant, and imidazolinone-tolerant canola cultivars represents a major advancement in weed management (Shaw, 1997).

Harker (1997) suggested that the use of glyphosate-tolerant and imidazolinone-tolerant canola cultivars may increase the risk of selecting for resistant weed populations because these herbicides are used extensively in other western Canada cropping systems. However, he indicates that glufosinate will not be registered for use other than in glufosinate-tolerant canola and thus represents a good tool for weed resistance management. Darwent (1994) cautioned that there is a potential for the transfer of the herbicide tolerant trait to weedy mustards.

Shaw (1997) suggested that farmers approach this technology with guarded optimism. He stated that there may be economic benefits from the herbicide-tolerant canola in the form of control of problem weeds, fewer field operations, and improved grain quality. However, growers are concerned with the delays in food safety approvals by other countries, weed resistance, and technology fees. Importantly, there does not appear to be an agronomic difference in yield potential when herbicide-tolerant and traditional canola cultivars are compared.

One interesting development with herbicide-tolerant canolas was the recall by Limagrain of two glyphosate-tolerant seed varieties, LG3315 and LG3295 (Leite, 1997). These varieties apparently had an unregistered construct of the glyphosate-tolerance gene which was discovered during a quality control program. Canola seed for an estimated 600,000 acres was recalled and destroyed just prior to the 1997 planting season (Rick Holm, Univ. of Saskatchewan, personal communication). This may result in more of the imidazolinone-tolerant and glufosinate-tolerant canola used than the glyphosate-tolerant cultivars.


Herbicide tolerant crops are not yet planted on a significant number of acres in North American. However, with the increasing availability of crops tolerant to herbicides such as glyphosate, glufosinate, imidazolinones, sethoxydim and sulfonylureas, they will become an important part of weed management strategies. Further, the agricultural chemical industry and seed companies see herbicide tolerant crops as an important source of profits. Farmers have extremely high expectations for weed control resulting from the herbicide tolerant crop systems. Importantly, the use of herbicide tolerant crops and appropriate herbicides is perceived to require lower management than conventional weed management strategies. Evidence suggests this is not the case.

Proponents suggest that there will be increased use of alternative management strategies as a result of the herbicide tolerant crop systems. However, given farmer expectations and marketing strategies, it is unlikely that alternative strategies will be used, and in fact, a greater reliance placed upon herbicides for weed control. When considered objectively, the use of herbicide tolerant crops as a weed management strategy does not differ greatly from current strategies.

Herbicide tolerant crops have some risks associated with the use of this technology. Weed resistance, misapplication, herbicide drift, and the need for timely application all must be considered as potential problems for herbicide tolerant crop technology. However, these same risks are associated with conventional weed management systems. Benefits of herbicide tolerant crops focus on the potential for consistent weed control in conservation tillage systems, the use of herbicides positioned as environmentally safe, and less crop injury. Whether the benefits are more important than the risks associated with herbicide tolerant crops will be determined by farmers and their acceptance of this technology. 


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Prepared by Mike Owen, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
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